The versatile herb is a rapid grower outdoors—after planting in early spring, it’s not unusual to see a lemon balm plant grow to over a foot tall (sometimes, even two!) in just a single season. For that reason, many gardeners choose to plant lemon balm in containers, whether grown indoors or out, in an attempt to control its invasive nature. Like many other herbs, lemon balm prospers best outdoors but can be grown successfully inside as well, as long as proper attention is paid to its unique needs. Lemon balm also has the tendency to bolt if exposed to too much water or not maintained properly. If you notice small flower buds beginning to form on your lemon balm plant, take care to pinch them off quickly. Once the plant blooms, the flavor of its leaves will change, causing them to taste bitter. To cook with your lemon balm, harvest the leaves as soon as the plant is established and putting out regular new growth. Take care to never remove more than about 25 percent of the plant’s mass at any one time.
Light
In both indoor and outdoor environments, lemon balm loves a sunny spot. However, unlike an outdoor lemon balm plant—which can tolerate a bit of shade—those planted indoors need ample sunlight, at least five to six hours a day. Place your herb containers near a windowsill that gets ample light throughout the day. If you notice your plant getting a lot of harsh direct rays or turning brown on its leaves, rotate it periodically to avoid burning.
Soil
Lemon balm plants prefer a slightly sandy, well-drained soil. Most basic potting soils will work just fine, but if you find your mixture is a bit dense or stays too moist between watering, combine it with sand or a drier soil varietal like a cactus mix. Additionally, the pH of your soil should be neutral to acidic, with a specific level between 6.7 to 7.3 providing the best environment. When selecting a container to house your lemon balm plant in, choose a pot with ample drainage holes to minimize the risk of the herb becoming waterlogged or the roots rotting. One made of clay or terracotta can also be helpful, as the porous material will help to wick any added moisture from the soil.
Water
When it comes to watering your lemon balm plant, always err on the side of under-watering, rather than over-watering. The reason: Like many herbs, lemon balm plants can recover easily from wilt (created by thirst) but will quickly die or bolt if watered too much. Your exact watering schedule will depend on the environment in your home and the plant’s sun exposure levels, but a good general rule of thumb is to water in small quantities (do not saturate) as soon as the first inch or so of soil in the pot has dried up.
Temperature and Humidity
When it comes to its indoor environment, the lemon balm plant is not particularly picky. In general, it’s best to keep your herb away from any drafty air that is too cold or too hot, such as in front of an air conditioner or next to a radiator. Additionally, lemon balm does not need a humid environment to thrive, so you won’t have to worry about increasing the moisture levels in the air with a humidifier.
Fertilizer
While lemon balm does not require additional feeding beyond what nutrition its soil provides, you can treat it with a light liquid fertilizer every few weeks to encourage growth. Keep in mind, fertilizing some herbs has been known to decrease the potency of their scent and/or flavor—if you’re growing lemon balm specifically for cooking, you can likely skip fertilizing it and still be just fine.
Propagating Lemon Balm
Lemon balm is what’s known as a self-seeding plant, meaning outdoors it spreads its seeds to further its growth (hence why some gardeners consider it invasive). However, most indoor growers won’t see their plants set seeds. Instead, it’s advised to discard your lemon balm plant if it bolts (goes to flower), in part because the taste and potency of the leaves will be diminished. It’s easy to start new plants from packaged seeds (they will germinate in a little over a week) or simply buy new seedlings at your local garden center.
How to Grow Lemon Balm From Seed
Lemon balm seeds need ample light in order to germinate and grow. To start your plant from seed, begin them in a shallow pot or dish, barely covering the seeds with more than a sprinkle of soil (this should be done about six to eight weeks before the last frost if you want to transplant the herb outdoors). Expose the dish to ample light and just enough water to keep them from drying out (its helpful to use a spray bottle instead of a watering can for this stage) and germination should take place in 10–14 days. Once the seedlings are large enough to handle easily, you can transplant them into a traditional pot with soil.
Common Diseases
Lemon balm has no serious pest issues, but can be susceptible to powdery mildew. To prevent it from developing, plant your lemon balm with ample room separating it from other plants to allow for good air circulation. If powdery mildew does develop, remove the infected leaves, and treat with a mild fungicide.
